Wellbores in the oil and gas industry are typically drilled using a drill string with a drill bit secured to its distal end. The drilled wellbore is subsequently completed by cementing a string of metal pipes connected end-to-end within the wellbore. Commonly called “casing,” such strings of metal pipes increase the integrity of the wellbore and provides a flow path between the earth's surface and selected subterranean formations. As used herein, the terms “pipes,” “casing,” and “pipelines” and variations thereof may be used interchangeably to cooperatively refer to the metal pipes or pipelines that line the walls of a wellbore.
The pipes that make up the casing may be made of plain carbon steel, stainless steel, or another material able to withstand a variety of forces, such as collapse, burst, and tensile failure. During the lifetime of a well, the casing is exposed to high volumes of materials and fluids required to pass through them, including chemically aggressive fluids. In harsh environments, however, the casing may be subject to corrosion that may affect its functionality. Timely and accurate detection of structural integrity problems such as cracks, pinholes, and corrosion is essential to reducing costs associated with wellbore intervention, since pulling a casing pipe out of a wellbore for further inspection and repairs and replacing can be a very expensive task.
Some wellbores include multiple concentric pipes or strings of casing secured within the wellbore with an innermost pipe that exhibits a relatively narrow diameter. As will be appreciated, the diameter of the innermost pipe limits the size of the monitoring and intervention system that can be deployed to monitor the integrity of all of the concentric pipes. With multiple concentric pipes, another problem is the ability to effectively monitor the outermost pipes from the innermost pipe, since any monitoring system has to be able to sense through a number of pipe layers, each of which may have developed distinct problems or defects.
Several different sensing methods have been proposed for detecting corrosion and other types of defects in pipelines, some of which have been applied to wellbore pipes used for extracting hydrocarbons. The most common method utilizes acoustic wave pulses and analysis of reflections from the surface of a pipe wall to image any defects. Electromagnetic inspection methods are also used for the same purpose, and are desirable since they allow an operator to sense beyond the first pipe, and thereby obtain measurements from second, third, or additional pipes beyond the third pipe. Existing pipe inspection methods, however, are either azimuthally sensitive and shallow or azimuthally insensitive and deep.